1. Field of the Invention
The present invention is directed to a postage meter machine of the type having a digital printer device with a guide plate and a transport arrangement with a conveyor belt for moving items to be printed past the printer device, and a control unit which generates print control signals for a printhead of the printer device, and having a letter sensor disposed in the guide plate preceding the printhead in the transport direction which supplies a sensor signal to the control unit to ready the printer device for printing.
2. Description of the Prior Art
Postage meter machines can be especially efficiently utilized for franking mail beginning with a moderate to high volume of letters or other postal matter to be mailed. Differing from other printers, a postage meter machine is suitable for processing filled envelopes, possibly also with very different formats. The terms letter, piece of mail or print medium as used herein include all types of envelopes or other print-receiving media. Postal matter, file cards, labels or self-adhesive paper tapes or similar material can be employed as the print medium.
Modern postage meter machines utilize fully electronic digital printer devices. For example, the postage meter machine T1000 of Francotype-Postalia AG & Co. employs a thermal printing unit. With this, it is possible to print arbitrary texts and special characters in the franking stamp printing region. A thermal transfer postage meter machine disclosed in U.S. Pat. No. 4,746,234 has a microprocessor and is surrounded by a secured housing that exhibits an opening for the delivery of a letter, thus the position and movement of the letter can only be indirectly determined. A mechanical letter sensor (micro-switch) communicates a print request signal to the microprocessor as an indication of the position of the letter during transport thereof through the machine. The necessity of precise timing as articles are transported at high speed through the machine results in a high outlay for adjusting triggering of the micro-switch, which is disadvantageous. The microprocessor controls the drive motors and an thermal transfer printing head based on the sensor output. An encoder communicates a signal derived from the transport of the thermal transfer inking ribbon (which moves in a path around a series of rollers) to the microprocessor as further information about the letter transport movement.
European Application 189 268 discloses a receptacle means for inking ribbon cassettes. The sidewall of the cassette has an opening through which a roller projects for seating the inking ribbon in order to receive the drive force therefrom, or to transmit the drive force to a friction roller that is coupled to an encoder disk. The inking ribbon speed approximately corresponds to that of the printed matter that is transported between the inking ribbon and the counter-pressure roller. Given slippage resulting in a lack of synchronization between the letter transport components and the thermal transfer inking ribbon transport components, the derived signal is no longer correct, which correspondingly influences the appearance of the print format.
U.S. Pat. No. 5,495,103 discloses an arrangement for printing on a piece of mail triggered in a time spacing with a time control. Printing is started with a reflected light barrier and with a timer, so that the print format can be exactly positioned on the piece of mail by the user. There is always the possibility, however, that a letter jam will arise given high letter transport speeds. The point of the print triggering shifts when the letter is moved faster. The stamp imprint thus no longer fits completely on the mailing. The required adjustment (made by setting potentiometer) that the user is supposed to undertake according to the printing speed is disadvantageous. The user must undertake an undefined number of trials for adjustment, and it is uncertain what setting of the potentiometers is most likely of success.
German PS 196 05 014 discloses an embodiment of a printer device (JetMail.RTM.) that, given a non-horizontal, approximately vertical letter transport, implements a franking print with an ink jet printhead arranged stationary in a recess behind a guide plate. A print sensor for recognizing the start of a letter is arranged before the recess for the ink jet printhead and collaborates with an incremental sensor. By means of pressure elements arranged on the conveyor belt, the letter transport is slip-free and the sensor signal derived during the transport is correct, which has a positive influence on the quality of the print format. Given thick letters, however, the letter's leading edge is not always squared, but can be more or less rounded, so that the start of the letter is not exactly detected.